A gate drive circuit for a semiconductor switching device (or a circuit which drives a semiconductor switching device) is a circuit which drives a gate terminal of the switching device (hereinafter the semiconductor switching device is also simply referred to as “switching device”). Such a gate drive circuit controls ON/OFF of the semiconductor switching device by applying a gate voltage to a gate terminal of, for example, a high withstand voltage switching device such as an insulated gate bipolar transistor (IGBT) called a power semiconductor. A general gate drive circuit includes a P-type transistor and an N-type transistor in an output unit. The P-type transistor operates when the switching device is turned ON from OFF and the N-type transistor operates when the switching device is turned OFF from ON. Specifically, when the switching device is turned OFF from ON, the gate current of the switching device is extracted.
In the above-described gate drive circuit, a reference potential of the power semiconductor switching device, that is the reference voltage on the output side of the gate driving circuit, is very high. Therefore, it is required to isolate a direct-current component between the primary side which is the input unit for receiving a control signal to the gate drive circuit and the output side (secondary side) of the gate drive circuit which drives the switching device. Such an element capable of isolating the direct-current component between the primary side and the secondary side of the element is called a DC isolation device. This is an element necessary to drive the switching device. Furthermore, the above-described electronic circuit element having the DC isolation function is used for isolating the logic ground and the RF ground, and is called a digital isolator (Patent Literature (PTL) 1). Particularly, to drive a power semiconductor switching device, an external isolated power supply is required which requires a very large gate drive circuit also called a gate drive system. Therefore, if it is allowed not only to isolate a gate signal but also to supply isolated power to the gate, the external isolated power supply is no longer required and the gate isolation circuit can be miniaturized.
As a representative signal transmission circuit which realizes the DC isolation function, a configuration which isolates signals via a wireless signal transmission unit such as a pulse transformer is known (for example, see PTL 2). However, the pulse transformer is so large that cannot be used for a semiconductor gate drive circuit. A technique has been proposed in which the gate drive circuit can be planalized and miniaturized to some extent by using a planer spiral inductors facing each other as a wireless signal transmitter, instead of the pulse transformer (for example, see PTL 3).
FIG. 14 illustrates an example of a digital isolator including a spiral inductor for a non-contact signal transmission unit disclosed in PTL 3. The digital isolator includes a transmission circuit chip 2041 in which a transmission circuit is formed, a transmission chip 2043 in which a transmission spiral inductor 2045 is formed, a reception chip 2044 in which a reception spiral inductor 2046 is formed, and a reception circuit chip 2042 in which a demodulation circuit is formed. The transmission circuit chip 2041 and the transmission chip 2043 are connected via a wire 2047. The reception circuit chip 2042 and the reception chip 2044 are also connected via the wire 2047. An input signal is modulated by the transmission circuit chip 2041 to a signal for wireless signal transmission, and is transmitted to the transmission spiral inductor 2045 of the transmission chip 2043. The transmission spiral inductor 2045 and the reception spiral inductor 2046 serve as coils. The transmission spiral inductor 2045 on the transmission chip 2043 and the reception spiral inductor 2046 on the reception chip 2044 are coupled by electromagnetic induction. Therefore, electric power (a current) transmitted to the transmission spiral inductor 2045 is transmitted to the reception spiral inductor 2046 electrically isolated from the transmission spiral inductor 2045. Electric power (a current) generated in the reception spiral inductor 2046 is restored by a receiving circuit on the reception circuit chip 2042, and is retrieved as an output signal. However, since wireless signal (electric power) transmission by such a planer spiral inductor uses electromagnetic induction coupling, there are problems in that transmission efficiency is bad and the withstand voltage is not high due to small air gap between the wires.
Therefore, a technique called electromagnetic resonance coupling (or electromagnetic field resonance coupling) has been reported as a wireless signal transmission technique realizing higher transmission efficiency and higher isolation voltage (for example, see PTL 1). Furthermore, as shown in FIG. 15, a power (signal) transmission apparatus using an open-ring electromagnetic resonance coupler as a wireless signal transmitter has been proposed (for example, see PTL 4),